1. Field of the Invention
This invention relates to an exposure control circuit for a camera, and more particularly to an exposure control circuit for a camera including a crystal oscillator.
2. Description of the Prior Art
It is known that a crystal oscillator has a very unstable starting characteristic but has a good stability after several minutes have lapsed since the start of power supply. Therefore, the crystal oscillator is used in a circuit which is normally energized such as a circuit in a watch.
If the crystal oscillator is used in an exposure control circuit of a camera, it becomes necessary to normally supply current to the circuit. Since the driving voltage of the crystal oscillator is desired to be 3 to 6V in an exposure control circuit including a light measuring circuit, an A-D converter and a digital control circuit, the current to be supplied to the crystal oscillator will be several milliampares. This will require a power source of large capacity, which is unfavorable in the photographic camera that is usually preferred to be as small as possible in size. If the power is supplied only when the pictures are taken, it will practically be impossible to take snaps at any time as desired.
Therefore, it has been considered impossible or very hard to use a crystal oscillator in a photographic camera. On the other hand, however, it is desirable that the high stability of the crystal oscillator be made use of in cameras in order to obtain a highly stable exposure control.